Yung Y.L., DeMore W.B. Photochemistry of Planetary Atmospheres
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236
Photochemistry
of
Planetary
Atmospheres
early
models
of
Triton's ionosphere
all
postulated that
the
principal energy source
is
precipitation
of
magnetospheric electrons.
The
required energy
flux is
about
a
factor
of
10
higher than that
of
solar
EUV
flux. We
show
that
there
is
another possibility:
the
chemistry
of C
atoms
and the
role
of its ion
C
+
in the
ionosphere.
There
are a
number
of
sources
of C
atoms,
the
most important
of
which
is
where
the ion
CO
+
is
derived from
Additional
contributions
to the
production
of C are
from
reactions (6.61)
and
where
the
quantum yield
of
(6.80)
at
Lyman
a is
0.4%.
The
excited
C
atom formed
in
(6.80)
is
readily quenched
by
C atoms react with N2 in a three-body reaction:
It
is
most
likely
that
CNN
would react
with
N:
followed
by
Note that
the net
result
of the
above reactions
is the
recombination
of N
atoms using
C
atoms
as a
catalyst:
This
is the
principal
chemical scheme
for the
recombination
of N
atoms.
The C
atoms
are
recycled
in
(III). Since
C has a low
ionization potential
(11.26
eV), most ions
will
transfer
to C in
reactions such
as
Conversely,
the
transfer
reaction
for
C
+
,
Figure
6.22 Temperature
and
number density
of the
atmosphere
of
Triton
as a
function
of
altitude
based
on
Voyager
2
data.
After
Yung
and
Lyons (1990).
Figure
6.23
Typical globally averaged number density
profiles
of
important minor
constituents
at
ionospheric
heights.
N
atom mixing ratio
at 400 km is
0.021.
The
C
density profiles were calculated
with
(dashed
line)
and
without (dot-dashed
line)
reactions
with
N2.
After
Strobel,
D. F., and
Summers,
M.
E.,
1995, Triton's upper
atmosphere
and
ionosphere,
in
Neptune
and
Triton (Tucson:
The
University
of
Arizona
Press),
p.
1107.
237
238
Photochemistry
of
Planetary
Atmospheres
is
extremely slow.
The
principal
sink
for
C
+
is
charge
transfer
to
CH4:
The
molecular ions produced
in
(6.9la)
and
(6.91b)
are
rapidly removed
by
dissocia-
tive
recombination. Since
the
mixing ratio
of
CH
4
falls
off
rapidly
with
altitude
due to
its
destruction
by
photolysis, reactions
(6.9la)
and
(6.91b)
result
in a
slow
loss
of
C
+
.
6.4.2 Photochemical Model
A
model atmosphere
of
Triton that
is
consistent with Voyager observations
is
presented
in
figure
6.22.
The
mixing ratios
of
CFU
and CO at the
surface
are 1.7 x
10~
4
and
1
x
10~
4
,
respectively.
H and
H
2
can
escape
at
almost thermal velocities. Heavier
atoms
can
also
escape
but at
much smaller
effusion
velocities.
The
concentrations
of the
neutral species computed
in the
model
are
presented
in
figure
6.23.
The
"parent
molecule"
of all
hydrocarbons,
CFU,
falls
off
rapidly with
altitude
due to its
loss
by
photodissociation.
The
rate
of
photolysis
peaks
near
the
surface.
The
optical depth near
Lyman
a for the
entire column
of
CPU
is on the
order
of
unity.
The
vertical
profile
of
CFU
computed
by the
model
is
consistent
with
Voyager
UVS
observations.
H, H2,
C2H4,
and
C
2
H
2
are the
most abundant products
Figure
6.24 Calculated
ion and
electron densities produced
by
solar radiation only
compared
with
RSS-measured
electron densities
for
winter,
16° N,
dusk (circles),
and
summer,
45° S,
dawn (crosses), occultations.
After
Lyons,
J. R. et
al.,
1992,
"Solar
Control
of the
Upper Atmosphere
of
Triton."
Science 256,
204-206.
Satellites
and
Pluto
239
of
CH4
photochemistry. However,
the
concentrations
of H and H2 are
limited
by
their
ability
to
escape
from
the
upper boundary.
The
higher hydrocarbons
are
removed
by
condensation
at the
surface. Conspicuous
by its
absence
is
ethane. There
are few
methyl
radicals produced
in the
reactions since there
is too
little
€2^2
to
drive
the
photosensitized
dissociation
of
CFLj.
CO
(not shown)
has
nearly
uniform
mixing
ratio
as it has
almost
the
same mass
as N2 and its
rate
of
destruction
in the
upper atmosphere
is
trivial
compared
with
the
rate
of
supply from
the CO ice on the
surface.
The
most important atomic species
in the
model
are N, O, and C,
produced pri-
marily
by
photolysis
of
N
2
and CO. The
most interesting consequences
of N and O
are the
production
of HCN and
CO2.
Removal
by
condensation
may be an
impor-
tant
source
of
CO
2
ice
that
has
been observed
on the
surface
of
Triton.
The
rate
of
deposition
on the
surface
is 1.3 x
10
5
cm~
2
s~
l
.
The
high abundance
of C
atoms
results
in
C
+
being
the
most abundant
ion in the
model
of the
ionosphere,
as
shown
in
figure
6.24.
The
C
+
concentration predicted
on the
basis
of
solar
photoionization
alone
is of the
correct
magnitude
to
account
for the
Voyager
RSS
experiment.
The
circles
and
crosses
denote
the
ingress (winter)
and
egress
(summer) observations,
re-
spectively. Concentrations
of
N
+
and
N2
+
are
about
one and two
orders
of
magnitude
smaller than
C
+
,
respectively.
In the
lower ionosphere there
are
more complex ions
such
as
HCO
+
and
H2CN
+
.
The
latter
ion is
believed
to be the
major
ion in the
atmosphere
of
Titan,
but its
concentration
is
small
in
Triton's ionosphere. Since
N2
is
the
major
gas in the
atmosphere,
the
bulk
of
photoionization results
in the
pro-
duction
of
N2
+
.
As
pointed
out in
section
6.4.1,
N
2
+
readily transfers charge
to
N
+
and
CO
+
,
and
eventually
to the
terminal
ion
C
+
.
The
unusually
large concentration
of
C
+
in the
model
is due to its
inability
to
transfer charge
to a
molecular ion.
This property
appears
to be
unique
to
C
+
,
since
the
other atomic ions
H
+
and
N
+
are
rapidly removed
by
charge transfer
to
molecular ions, followed
by
dissociative
recombination.
6.5
Pluto
Pluto,
the
ninth
planet
in the
solar system,
was
discovered
in
1930.
Our
present
knowledge
of
Pluto
is at the
level
of
astronomy rather than planetary science.
The
planet
has not
been visited
by any
spacecraft.
The
orbit
of
Pluto
is
highly
eccentric
(e
=
0.25)
and
inclined
(i =
17°),
with
an
orbital period
of 248 yr. The
semimajor
axis
is
39.6
AU, but
because
of its
high eccentricity
its
perihelion
at
29.7
AU is
inside
the
orbit
of
Neptune.
The
aphelion
is at
49.5
AU,
near
the
edge
of the
Kuiper
Belt.
The
solar insolation
at
Pluto varies
by
about
a
factor
of 3
between aphelion
and
perihelion.
The
disk
of
Pluto cannot
be
resolved
in
telescopic observations and,
to
complicate matters, Pluto
has a
satellite, Charon, that cannot
be
resolved from
the
planet. Thus,
for a
long time
the
radius
and the
density
of the
planet remained
uncertain
by
large factors. However,
a
series
of
observations
of the
mutual
eclipse
events
of
Pluto
and
Charon
(a
once
per
century event that occurred between 1985
and
1990)
has
yielded
highly
reliable orbital
and
physical parameters
for the
system.
Note
that
the
radius
of
Pluto,
1151
km, is
less
than
Triton's 1360
km. The
density
of
the
Pluto-Charon system
is
2.03
gcm~
3
,
a
value
that
is
typical
of an icy
body
with
a
rocky
core.
240
Photochemistry
of
Planetary
Atmospheres
Figure 6.25
A
comparison
of a
continuum-adjusted spectrum
of
Pluto
and a
model
spectrum
based
on the
indicated
mixture
of
ices over
the
wavelength
range
of 1.4 to 2.4
/urn.
The
model
is an
intimate
mixture
of
the
components rather than
the
expected molecular mixture.
After
Owen,
T. C. et
al.,
1993,
"Surface ices
and the
atmospheric composition
of
Pluto,"
Science
261, 745.
6.5.1 Surface
The
high
geometric albedo
of
Pluto suggests that
the
surface
of the
planet
is
covered
by
a
white frost. Figure 6.25 gives
the
infrared solar reflectance spectrum
of the
surface
of
Pluto. Comparison with
a
model spectrum shown also
in
figure 6.25 suggests that
the
bulk composition
of the
surface frost
is
N
2
,
with
trace
amounts
of
CPU
and CO.
This composition
is
similar
to
that
of the
surface
of
Triton with
the
exception
of
CC«2,
which
is
present
on
Triton
but not on
Pluto.
The
surface material
of
Pluto consists
of
98%
N
2
,
1.5%
CH
4
,
0.5%
CO, and
less
than 0.07%
CO
2
.
For
comparison,
the
surface
of
Triton consists
of 99%
N
2
,
0.05%
CH
4
,
0.1%
CO, and
0.2%
CO
2
.
The
overall
chemical composition
of the
minor constituents
is
more reducing
on
Pluto than
on
Triton.
No
H
2
O
ice has
been
detected
on
either Pluto
or
Triton,
but its
presence
has
been established
on the
surface
of
Pluto's satellite Charon.
6.5.2
Atmosphere
The
existence
of an
atmosphere
on
Pluto
may be
inferred from
the
presence
of
ices
on
the
surface.
A
minimum atmosphere would
be
given
by the
vapor pressure
of
N
2
at
the
temperature
of the
surface.
The
best determination
of the
surface temperature
from
millimeter wave thermal emissions
of the
planet
is
30^44
K,
with
a
preferred
range
of
35-37
K.
However,
the
variation
of
vapor pressure
of
N
2
,
CH^
and CO
with
temperature over this range
is
large,
as
shown
in figure
6.21.
For
example,
at
30 K the
vapor pressure
of
N
2
is
about
0.1
/xbar,
but at 44 K the
vapor
pressure
exceeds
500
fibar.
Thus,
the
atmospheric
pressure
is
poorly constrained
by the
thermal
data.
The
correct
order
of
magnitude
of
Pluto's
atmosphere
has
been
deduced
from
the
occultation
of a
star
by
Pluto
in
1988.
The
atmosphere revealed
its
presence
by the
gradual
rather than abrupt dimming
of the
star's
light.
The
simplest interpretation
of
this
beautiful
experiment
is to
assume
an
isothermal, spherical,
and
hydrostatic
atmosphere.
The
half-light
level
of the
light curve
occurred
at
1214
± 20 km, and at
this
altitude
the
atmospheric
scale
height
was
59.7
± 1.5 km.
Assuming that
the
bulk
Satellites
and
Pluto
241
Figure
6.26 Model temperature
profiles
for the
atmosphere
of
Pluto,
with
the
surface
temperature
TO
= 35 K,
surface pressure
po
= 3
nbar,
ycH
4
=
3%.
"N
2
=
97%,"
"Ar
=
97%,"
and
"CO
=
97%"
denote atmospheres
dominated
by N2, Ar, and CO,
respectively.
After
Strobel
D. F. et
al.,
1996,
"On the
vertical
thermal
structure
of
Pluto's atmosphere," Icarus
120,
p.
266.
atmosphere
is N2, the
inferred temperature
is
117
K. The
pressure
is
0.78
/^bar
at the
half-light
level. There
is
general agreement
on the
interpretation
of the
light-dimming
curve from
first
contact
to the
half-light
level.
A
careful examination
of the
extinction data reveals
that
the
light-dimming curve
has two
distinct
slopes.
The
isothermal model
that
provides
a
satisfactory
fit to the
data
fails
as the
light
ray
from
the
star gets close
to the
surface
of
Pluto. There
are
two
models that
can
account
for the
change
of
slope
in the
data.
One
model postulates
the
presence
of an
aerosol layer
with
vertical optical depth exceeding
0.19
at
0.64
/j.m
immersed
in an
isothermal atmosphere.
An
alternative interpretation
is
that
the
discontinuity
near
the
half-light
level
is
caused
by a
large temperature gradient near
the
surface
of
Pluto.
At the
moment
the
controversy
is not
resolved.
We
briefly
comment
on
the two
models.
The first
model based
on an
isothermal atmosphere
of
117
K
is
obviously incorrect near Pluto's surface since
it
cannot
exceed
44 K
based
on the
vapor pressure
of
N2.
Therefore,
the
existence
of a
thermal gradient near
the
surface
is
not
in
doubt, especially when compared
with
the
temperature profile
of the
atmosphere
of
Triton.
But the
thermal gradient model requires almost
all the
heat
be
deposited
in
the
first
scale height
of the
atmosphere
to
produce
the
necessary gradient
of 10
K/km.
Ironically,
the
most plausible solution
(in our
opinion)
to the
heating problem
is to
postulate aerosol heating! Thus,
the
true solution
may
involve
a
combination
of
these
two
vastly
different
models. This range
of
uncertainty
is
illustrated
by the
thermal
models shown
in figure
6.26.
There
has
been
no
quantitative
modeling
of the
photochemistry
of
Pluto's atmo-
sphere.
We
expect
the
essential features
of the
hydrocarbon, nitrogen,
and
oxygen
chemistry
to be
similar
to
that
of
Triton. Since
the
abundances
of
CPU
and CO are an
242
Photochemistry
of
Planetary
Atmospheres
order
of
magnitude higher
on
Pluto's surface than
on
Triton's surface,
we
expect that
the
important chemical
processes
occur
at
higher altitudes, well above
the
region
of
condensation
for the
photochemical products
at the
surface. Although Pluto
is
farther
away
from
the sun
than Triton,
it has a
more extended atmosphere (see section
6.5.3)
with
an
optical
cross
section that
is
much larger
than
the
geometric
cross
section
of
the
planet. Thus,
the EUV
solar radiation received
by the two
atmospheres
may be
comparable.
6.5.3 Extended Atmosphere
Due to the low
gravity
at the
surface
of
Pluto,
the
atmosphere
is
believed
to
extend
well
above
the
surface. Indeed,
the
optical path
of
unity
for
Lyman
a may
occur
at two
Pluto
radii from
the
center
of the
planet.
In
this
case
the
total
flux of EUV
radiation
received
by the
planet
is
about four times that intercepted
by the
geometric
cross
section
of the
planet. Early researchers recognized
the
possibility
of
hydrodynamic
escape
of CH4
from Pluto, resulting
in
catastrophic
loss
of
atmosphere
and
surface
material
(as in a
comet). However, later studies demonstrated that
the
escape
may be
limited
by the
availability
of
solar
EUV
energy.
The
maximum
escape
flux is of the
order
of
10
10
cm~
2
s"
1
.
There
are no
observations
to
date
to
verify
this intriguing
hypothesis.
Escape
rates
of
this magnitude
are at
least
two
orders
of
magnitude greater than
the
corresponding
escape
rates from
the
atmospheres
of the
terrestrial planets. Higher
rates
of
escape
from
the
inner planets
are
believed
to
have occurred
close
to the
time
of
origin
of
their atmospheres.
A
similar mechanism, hydrodynamic
escape,
could
have
operated
on the
terrestrial planets. Thus,
an
understanding
of the
evolution
of
the
atmosphere
of
Pluto
may
provide insights into
the
early evolution
of
planetary
atmospheres
in the
inner solar system.
6.6
Unsolved
Problems
We
list
a
number
of
outstanding
unresolved
problems
related
to the
small
bodies
in
the
solar
system:
1
.
Is
there
a
residual atmosphere
on the
nightside
of
lo?
What
is it
made
of?
2.
What
is the
nature
of the
interaction between
lo's
atmosphere
and the
Jovian
mag-
netosphere?
3.
What
is the
ultimate
fate
of
higher hydrocarbons (derived
from
CH4
photochemistry)
on
the
surface
of
Titan, Triton,
and
Pluto?
4. Is the
origin
of CO on
Titan primordial
or
photochemical?
5.
What
is the
chemical scheme that produces
CHsCN
in the
atmosphere
of
Titan?
In
reactions
what
is the
branching ratio
&5u,/fc5i
a
?
6.
What
is the
fate
of the
condensed hydrocarbons
and
nitrites
when exposed
to
pro-
longed
ultraviolet radiation
in the
stratosphere
of
Titan
or the
surface
of
Triton
and
Pluto?
Satellites
and
Pluto
243
7. Why is
there
so
little
CO
relative
to
ChU
on
Triton
and
Pluto when observations
show that
CO is the
dominant
form
of
carbon
in the
molecular clouds?
8. Is
there
an
aerosol layer
or an
extremely steep thermal gradient near
the
surface
of
Pluto?
9.
What
are the
abundances
of the
photochemically
produced species
in the
atmosphere
of
Pluto?
10.
Is
there
hydrodynamic
escape
of
gases
from
the
atmosphere
of
Pluto?
11.
Why are the
abundant noble
gases
(e.g.,
Ar
and Ne)
absent from
the
atmospheres
of
the
small bodies?
7
Mars
7.1
Introduction
Mars
has
been
extensively
studied
by a
series
of
spacecraft since
the
dawn
of the
space
age:
by
Mariners
4, 6, 7, and 9
(1965-1972),
Mars
2
through
6
(1971-1974),
and
the two
Viking Landers
and
Orbiters
in
1976.
The
knowledge from spacecraft
is
supplemented
by
ground-based observations.
The
essential aspects
of
Mars
are
summarized
in
table
7.1.
It is a
smaller planet
than
Earth;
the
radius
and
mass are, respectively,
53% and 11% of
Earth.
The
surface
gravity
is
3.71
m
s~
2
,
compared
with
the
terrestrial value
of
9.82
m
s~
2
.
The
physical
properties
and
composition
of the
Martian atmosphere
are
summarized
in
tables
7.1
and
7.2; isotopic composition
is
given
in
table 7.3.
An
example
of how
this knowledge
is
obtained
is
illustrated
in figure
7.1,
showing
the
mass spectrum obtained
by the
mass
spectrometer experiment
on
Viking.
The
bulk
atmosphere
is
composed
of
CC>2,
with
small amounts
of N2 and Ar and a
trace
amount
of
water vapor. Located
at
1.52
AU
from
the
sun,
the
mean insolation
at
Mars
is
about
half
that
of
Earth.
As a
result,
it is a
colder planet,
with
mean surface
temperature
of 220
K,
too
cold
for
water
to flow on the
surface
in the
current epoch.
The
lack
of an
ocean results
in an
arid
and
dusty climate.
The
obliquity
of
Mars
is
25.2°,
close
to the
terrestrial value
of
23.5°;
however, Mars
has an
eccentric orbit,
with
eccentricity
of
0.093.
The
ratio
of
incident
solar radiation
at
perihelion
to
aphelion
is
1.45.
The
large seasonal
variation
in
heating
is
believed
to be
responsible
for the
spectacular global dust storms that
can be
observed
from
Earth
and
have inspired
imaginative
but
erroneous theories about their origin.
The
polar regions
of
Mars
can
244
Mars
245
Table
7.1
Astronomical
and
atmospheric
data
for
mars
Radius
(equatorial)
3394.5
km
Mass
6.4185
x
10
2
-
1
kg
Mean
density
3.9335
g
cm~
3
Gravity
(surface, equator)
3.711
m
s~
2
Semimajor
axis
1.52366
AU
Obliquity
(relative
to
orbital plane) 25.19"
Eccentricity
of
orbit
0.0934
Period
of
revolution
(Earth days)
686.98
Orbital
velocity 24.13
km
s"
1
Period
of
rotation
24*
37™
22.663
s
Atmospheric
pressure
at
surface
5.6
mbar
Mass
of
atmospheric column
1.50
x
10~
2
kg
cm"
2
Total
atmospheric mass 2.17
x
10
16
kg
Equilibrium
temperature
216 K
Surface
temperature
220 K
Annual
variation
in
solar
insolation
3
1.45
Atmospheric
scaJe
height
(7"
= 210 K)
10.8
km
Adiabatic
lapse rate (dry)
4.5 K
km~'
Atmospheric lapse
rate
h
2.5 K
km"
1
Escape velocity
5.027
km s"'
Taken from
Kieffer,
H. H. et
al.
(1992).
a
Based
on the
ratio
of
aphelion
lo
perihelion distance squared.
b
Mean
observed value, variable.
be as
cold
as 125 K, so
CC>2
will
condense
as
frost
on the
surface.
In
fact,
according
to the
Leighton-Murray
model,
this
is
what determines
the
pressure
of the
atmosphere.
Figure
7.2
shows
the
seasonal pressure variations
at the
Viking lander sites
for 3.3
Mars
years from 1976. Note
that
the
magnitude
of the
pressure changes
is of the
order
of
20%, compared
to the
maximum change
of 1% on the
surface
of
Earth.
The
most fascinating
aspect
of the
photochemistry
of
Mars
is the
discovery
by
McElroy
in
1972
that
there
is
self-regulation
of the
oxidation state
of the
atmosphere
Table
7.2
Chemical composition
of the
atmosphere
of
Mars
Gas
CO
2
N
2
40
AT
0
2
CO
H
2
O
36
Ar+
38
Ar
Ne
Kr
Xe
0
3
Abundance
0.9532
0.027
0.016
1.3
x
Ifr
3
7.0
x
10"
4
3.0
x
10"
4
5.3 x
10~
6
2.5
x
10'
6
3.0 x
10~
7
8.0
x
to"
8
3.0 x
10~
8
Reference
(1)
(1)
(1)
(1)
(1)
(2);
(1)
(1)
(1)
(1)
(3);
and
remarks
variable
variable
(1)
Owen
et al.
(1977);
(2)
Farmer
and
Doms (1979);
(3)
Earth
(1974).